In my experience you're correct. I have a 5 gallon restaurant container that typically brine a 20 - 22ish lb bird in. 1.5 gallons of brine usually does me just fine. I think I'd worry more about the right concentrate of the water and salt. If it was me, I'd put the "sealed" birds in the bucket and measure how much water it takes to submerge them and build your brine based off that data. Remember a small portion will go into the cavity once unsealed, but probably not enough to make a noticeable difference.